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Darwish" Subject: [PATCH v1 1/1] Documentation: real-time: Add kernel configuration guide Date: Thu, 5 Mar 2026 21:50:12 +0100 Message-ID: <20260305205023.361530-2-darwi@linutronix.de> In-Reply-To: <20260305205023.361530-1-darwi@linutronix.de> References: <20260305205023.361530-1-darwi@linutronix.de> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: quoted-printable Add a configuration guide for real-time kernels. List all Kconfig options that are recommended to be either enabled or disabled. Explicitly add a table of contents at the top of the document, so that all the options can be seen in a glance. Whenever appropriate, link to other kernel guides; e.g. cpuidle, cpufreq, power management, and no_hz. Add a summary at the end of the document warning users that there is a no "one size fits all solution" for configuring a real-time system. Signed-off-by: Ahmed S. Darwish --- Documentation/core-api/real-time/index.rst | 1 + .../real-time/kernel-configuration.rst | 297 ++++++++++++++++++ 2 files changed, 298 insertions(+) create mode 100644 Documentation/core-api/real-time/kernel-configuration.r= st diff --git a/Documentation/core-api/real-time/index.rst b/Documentation/cor= e-api/real-time/index.rst index 7e14c4ea3d59..2376c7754d8e 100644 --- a/Documentation/core-api/real-time/index.rst +++ b/Documentation/core-api/real-time/index.rst @@ -14,3 +14,4 @@ the required changes compared to a non-PREEMPT_RT configu= ration. theory differences architecture-porting + kernel-configuration diff --git a/Documentation/core-api/real-time/kernel-configuration.rst b/Do= cumentation/core-api/real-time/kernel-configuration.rst new file mode 100644 index 000000000000..8f47f2e2aaf7 --- /dev/null +++ b/Documentation/core-api/real-time/kernel-configuration.rst @@ -0,0 +1,297 @@ +.. SPDX-License-Identifier: GPL-2.0 + +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D +Real-Time Kernel configuration +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D + +.. Add a ToC so that all options can be seen in a glance +.. contents:: Table of Contents + :depth: 3 + :local: + +Introduction +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D + +This document lists the kernel configuration options that might affect a +real-time kernel's worst-case latency. It is intended for system integrat= ors. + +Configuration options +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D + +``CONFIG_CPU_FREQ`` +------------------- + +:Expectation: enabled +:Severity: *high* + +The CPU frequency scaling subsystem ensures that the processor can operate +at its maximum supported frequency. While, in general, bootloaders are +tasked with setting the CPU clock to the highest speed on boot, some do +not. It is thus desirable to keep this option enabled. + +.. caution:: + + A real-time kernel is not about being "as fast as possible", however + real-time requirements may demand that the CPU is clocked at a + particular speed. + +``CONFIG_CPU_FREQ_DEFAULT_GOV_PERFORMANCE`` +------------------------------------------- + +:Expectation: enabled +:Severity: *high* + +Real-Time workloads expect a fixed CPU frequency during execution. Using +the performance governor is an easy way to achieve that purely from kernel +configuration. + +This is not a blanket rule. Some setups might prefer to clock the CPU to +lower speeds due to thermal packaging or other requirements. The key is +that the CPU frequency remains constant once set. + +``CONFIG_CPU_IDLE`` +------------------- + +:Expectation: enabled +:Severity: *info* + +CPU idle states (C-states) allow the processor to enter low-power modes +during periods of inactivity. Very-low CPU idle states may require +flushing the CPU caches and lowering or disabling the clocking. This can +lower power consumption, but it also increases the entry and exit latency +from such states. + +While disabling this option eliminates cpuidle-related latencies, doing so +can significantly impact hardware longevity, warranty, and thermal +behavior. Users should cap the maximum C-state to C1 instead. For ACPI +platforms, this can be achieved by using the boot parameter [1]_:: + + processor.max_cstate=3D1 + +Higher C-states can be acceptable depending on the user workload's latency +requirements. For ACPI-based platforms, use the ``cpupower idle-info`` +command to inspect the available idle states. + +For more information, please see: + +- ``linux/tools/power/cpupower`` +- :doc:`/admin-guide/pm/cpuidle` +- :doc:`/admin-guide/pm/index` + +``CONFIG_DRM`` +-------------- + +:Expectation: disabled +:Severity: *info* + +GPU-accelerated workloads can share system resources with the CPU, +including last-level cache (LLC) and memory bandwidth. Modern integrated +GPUs optimize graphics performance at the expense of CPU determinism. + +Examples of affected platforms: + +- Intel processors with integrated graphics (Gen9 and later) +- AMD APUs with Radeon Graphics +- Xilinx Zynq UltraScale+ MPSoC EG/EV series + +If graphics workloads must run alongside real-time tasks, users must +conduct thorough stress testing using tools like ``glmark2`` while +measuring the overall system latency. + +For more information, please check: + +- :doc:`Regarding hardware (System memory and cache) ` +- :doc:`/filesystems/resctrl` +- `Real-Time and Graphics: A Contradiction?`_ + +``CONFIG_EFI_DISABLE_RUNTIME`` +------------------------------ + +:Expectation: enabled +:Severity: *medium* + +EFI is the standard boot and firmware interface for multiple +architectures. EFI runtime services provide callback functions to be +called from the kernel; e.g., as utilized by (``CONFIG_EFI_VARS*``) or +(``CONFIG_RTC_DRV_EFI``). For the former, the kernel calls into EFI to +update the EFI variables. + +Calling into EFI means invoking firmware callbacks. During such +invocations, the system might not be able to react to interrupts and will +thus not be able to perform a context switch. This can cause significant +latency spikes for the real-time system. + +``CONFIG_PREEMPT_RT`` enables this option by default. If this option is +disabled during the kernel build, pass the following boot parameter [1]_:: + + efi=3Dnoruntime + +There is ongoing `development work`_ to allow EFI variables access for a +real-time Linux system. + +``CONFIG_NO_HZ`` / ``CONFIG_NO_HZ_FULL`` +---------------------------------------- + +:Expectation: disabled +:Severity: *medium* + +Tickless operation can increase kernel-to-userspace transition latency due +to the extra accounting and state book-keeping. + +*Guidance by real-time workload type:* + +- For periodic workloads; e.g., control loops executing every 100 =C2=B5s,= avoid + ``NO_HZ`` modes. Consistent kernel ticks are preferable. + +- For computation-intensive workloads; e.g. extended userspace execution, + ``NO_HZ_FULL`` may be beneficial. In such cases, users should offload + the kernel housekeeping to dedicated CPUs and isolate compute cores. + +See also :doc:`/timers/no_hz`. + +``CONFIG_PREEMPT_RT`` +--------------------- + +:Expectation: enabled +:Severity: **fatal** + +This option must be enabled, or the resulting kernel will not be fully +preemptible and real-time capable. + +``CONFIG_TRACING`` (and tracing options) +---------------------------------------- + +:Expectation: enabled +:Severity: *info* + +Shipping kernels with tracing support enabled (but not actively running) +is highly recommended. This will allow the users to extract more +information if latency problems arise. + +.. caution:: + + Users should *not* make use of tracers or trace events during production + real-time kernel operation as they can add considerable overhead and + degrade the system's latency. + +Non-performance CPU frequency governors +--------------------------------------- + +:Expectation: disabled +:Severity: *medium* + +To ensure reproducible system latency measurements, disable the +non-``PERFORMANCE`` CPU frequency governors when possible. This avoids the +risk of unknown userspace tasks implicitly or explicitly setting a +different CPU frequency governor, and thus achieving different latency +results across the system's runtime. + +If disabling other frequency governors is not an option, then +``CPU_FREQ_DEFAULT_GOV_USERSPACE`` should be enabled. In that case, users +should set a *stable* CPU frequency setting during the system runtime, as +changing the CPU frequency will increase the system latency and affect +latency measurements reproducibility. If a lower CPU frequency is desired, +then ``CPU_FREQ_DEFAULT_GOV_POWERSAVE`` should be set. + +The ``ONDEMAND`` CPU frequency governor should *not* be enabled in a +real-time system since in dramatically affects determinism depending on the +workload. + +For more information, please check :doc:`/admin-guide/pm/cpufreq`. + +Kernel Debug Options +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D + +Most kernel debug options add runtime overhead that increases the +worst-case latency. + +.. caution:: + + During development and early testing, users are encouraged to run their + real-time workloads and peripherals with lockdep and other kernel debug + options enabled, for a considerable amount of time. Such workloads + might trigger kernel code paths that were not triggered during the + internal Linux real-time kernel development, thus helping to uncover any + real-time latency issues in the kernel. + +Problematic debug options +------------------------- + +``CONFIG_LOCKUP_DETECTOR`` +^^^^^^^^^^^^^^^^^^^^^^^^^^ + +Severity: *high* + +The lockup detector creates kernel timer callbacks that execute every few +seconds, in hard-IRQ context, even on real-time kernels. These periodic +interrupts can cause latency spikes. + +Users should use hardware watchdogs instead, which will provide a similar +functionality without the software-induced latency. + +``CONFIG_PROVE_LOCKING`` +^^^^^^^^^^^^^^^^^^^^^^^^ + +Severity: *high* + +Proving the correctness of all kernel locking adds substantial overhead +and significantly increases worst-case latency. + +Allowed kernel debug options +---------------------------- + +Kernel debug options which are not included in this list should be enabled +with caution, after extensive auditing of their impact on system latency. + +``CONFIG_DEBUG_ATOMIC_SLEEP`` +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +This sanity check catches common kernel programming errors with +a tolerable latency cost. + +``CONFIG_DEBUG_BUGVERBOSE`` +^^^^^^^^^^^^^^^^^^^^^^^^^^^ + +This improves the debugging capabilities without affecting normal +operation latency. + +``CONFIG_DEBUG_FS`` +^^^^^^^^^^^^^^^^^^^ + +This is safe to include in real-time kernels, *provided that debugfs is +not accessed during production runtime*. + +``CONFIG_DEBUG_INFO`` +^^^^^^^^^^^^^^^^^^^^^ + +This increases the kernel image size but has no latency impact. It is +also essential for meaningful crash dumps and profiling. + +``CONFIG_DEBUG_KERNEL`` +^^^^^^^^^^^^^^^^^^^^^^^ + +Meta-option which allows debug features to be enabled. This configuration +option has no runtime impact, but be aware of any debug features that it +may have allowed to be enabled. + +Summary +=3D=3D=3D=3D=3D=3D=3D + +There is no "one size fits all" solution for configuring a real-time Linux +system. Beginning with the system real-time requirements, integrators +must consider the features and functions of the system's hardware, kernel, +and userspace. All such components must be properly configured in order +to establish and constrain the system's maximum latency. + +With that in mind, any false real-time kernel configuration could cause a +new maximum latency that shows up at the wrong time and is catastrophic +for the real-time system's latency. + +References +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D + +.. [1] See :doc:`/admin-guide/kernel-parameters` + +.. _development work: https://lore.kernel.org/r/20260205115559.1625236-1-b= igeasy@linutronix.de + +.. _Real-Time and Graphics\: A Contradiction?: https://web.archive.org/web= /20221025085614/https://linutronix.de/PDF/Realtime_and_graphics-acontradict= ion2021.pdf --=20 2.53.0